Sulfobutylated lignin/polyaniline hybrid cathodes to enhance redox stability for lithium-ion batteries

Abstract

The global transition toward sustainable energy systems has intensified the search for electrode materials that combine high electrochemical performance with environmental compatibility. Conventional inorganic cathodes, while effective, rely on scarce or toxic elements and present challenges in recyclability and cost. Organic polymers, by contrast, offer structural tunability, lightweight composition, and potential renewability, but their practical application has been hindered by poor conductivity, limited cycling stability, and thermal degradation. Lignin, the most abundant aromatic biopolymer on Earth and a major by‑product of the pulp and paper industry, represents a promising candidate for sustainable electrode design. Its aromatic backbone, rich in functional groups, provides opportunities for chemical modification and integration with conductive polymers. This study explores the rational modification of lignin through sulfobutylation and its subsequent incorporation into polyaniline (PANI) composites to develop bio‑based cathode materials for lithium‑ion batteries. The primary objective of this research was to establish a direct structure–property relationship between lignin’s charge density, polymer integration, and electrochemical stability. Sulfobutylated lignin–polyaniline (SBLP) composites were synthesized with varying degrees of sulfobutylation, yielding SBLP2.0 and SBLP2.5 formulations. The –SO₃⁻ groups introduced by sulfobutylation acted as fixed ionic charge sites, stabilizing the emeraldine form of PANI and promoting Li⁺ transport. In addition, π–π stacking and hydrogen bonding interactions between lignin and PANI enhanced structural flexibility, facilitating chain rearrangement during charge/discharge cycles. These synergistic effects were hypothesized to improve conductivity, reduce polarization, and strengthen pseudocapacitive contributions relative to pristine polyaniline. Comprehensive characterization was performed to validate these hypotheses. Thermogravimetric analysis (TGA) confirmed improved thermal stability of the composites compared to pristine PANI, addressing a critical limitation of organic electrode materials. X‑ray diffraction (XRD) and Brunauer–Emmett–Teller (BET) surface area analysis revealed enhanced porosity and structural order in the SBLP composites, supporting improved ion accessibility. Electrochemical evaluation included cyclic voltammetry (CV), galvanostatic charge–discharge cycling, and rate capability testing under sequential current densities (C/20, C/10, C/2, and return to C/20). The CV profiles of SBLP composites exhibited higher peak currents and broader integrated areas, indicating increased charge‑storage capacity and faster kinetics. Galvanostatic cycling demonstrated clear performance differences: SBLP2.5 delivered the highest initial discharge capacity (≈220 mAh g⁻¹), while SBLP2.0 achieved superior structural reversibility, recovering ~90% of its initial capacity after rate cycling. Pristine PANI, by contrast, suffered pronounced capacity fading and poor recovery (~70%), consistent with its limited conductivity and susceptibility to structural decay in LiPF₆ electrolyte. Quantitative comparison of cycling metrics further highlighted the balanced performance of SBLP2.0, which achieved 80% capacity retention and 95% coulombic efficiency, outperforming both pristine PANI (56% retention, 70% efficiency) and SBLP2.5 (64% retention, 86% efficiency). These results confirm that while higher sulfobutylation increases initial capacity by introducing more ion‑accessible sites, excessive modification can induce structural disorder, segmental swelling, and interfacial strain that compromise long‑term stability. Thus, an optimal degree of sulfobutylation is critical for maximizing both performance and durability. Overall, this thesis demonstrates that rational chemical modification of lignin, combined with conductive polymers, can yield sustainable and electrochemically active cathode materials for next‑generation lithium‑ion batteries. The findings advance the understanding of how lignin’s charge density and polymer interactions govern electrochemical behavior, providing a framework for designing bio‑based composites with tailored properties. Future work will explore alternative lignin derivatives, varied sulfobutylation degrees, and full‑cell configurations to enhance electrochemical durability and scalability further further. By bridging renewable materials chemistry with energy storage technology, this research contributes to the development of greener, more sustainable batteries that align with global energy and environmental goals.